Magnetic recording media are widely used in various applications such as recording tapes, video tapes, floppy disks and the like. The magnetic recording medium basically comprises a non-magnetic support and magnetic layers formed thereon, the magnetic layers containing a ferromagnetic powder dispersed in a binder.
Given their intended applications, the magnetic recording media necessarily must be well-developed in levels of performance characteristics such as electromagnetic characteristics, running durability and running properties. In particular, with the recent popularization of 8-mm video tape recorders, the video tapes must be high in video output and have excellent ability to reproduce original images, and must also display excellent electromagnetic characteristics in general.
Various methods for improving the electromagnetic characteristics of the magnetic recording media are known including methods involving direct manipulation of the characteristics of the ferromagnetic powder. For instance, the ferromagnetic powders are gradually pulverized to a greater extent so that higher density recording is possible, and, as another approach, the raw materials for the ferromagnetic powders are also shifted from iron oxide to iron oxide modified with a diverse metal such as cobalt. Further, ferromagnetic metals such as iron, nickel and cobalt or alloys containing these elements have recently come into use.
By using the above conventional methods to improve ferromagnetic powders, magnetic recording media having good electromagnetic characteristics can be theoretically obtained. In actual practice, however, it is difficult to produce magnetic recording media which are improved in electromagnetic characteristics to a degree commensurate to any improvement provided in the ferromagnetic powder itself. This lack of carryover in improvement is due to the following reason. Namely, the dispersibility of the ferromagnetic powder into the binder tends to decrease as the ferromagnetic powder is more extensively pulverized, and also as a function of the type of ferromagnetic powder used. For example, the dispersibility tends to decrease in order of .gamma.-iron oxide, .gamma.-iron oxide coated with cobalt and ferromagnetic metal powders. Consequently, the situation is occasionally encountered wherein the dispersed state of the ferromagnetic powder in the magnetic layer is deteriorated in manner conversely to the improvement of the ferromagnetic powder, so that the excellent characteristics of the ferromagnetic powder itself are not fully exhibited in the finished magnetic recording medium.
In order to improve the dispersed condition of the ferromagnetic powder in a binder, a method is also known in which dispersion by kneading is performed for a prolonged period of time in preparing a magnetic layer. However, considerable shear stresses act on the ferromagnetic powder during such dispersion by kneading, so that adverse effects due to this shearing action are correspondingly increased with prolongation of such kneading time. For this reason, the characteristics of the ferromagnetic powder are sometimes lost when such kneading is used. In addition, the problem of diminished working efficiency is encountered due to the requirement of an extended period of time required for the production of the magnetic recording medium.
Due to above state of affairs, methods for effectively dispersing the ferromagnetic powder as described above without significantly changing typical manufacturing methods for producing the magnetic recording media have been studied. Such methods include a method of using ferromagnetic powders surface treated with surface treating agents such as silane coupling agents, a method of pretreating ferromagnetic powders with components for improving the dispersibility of the ferromagnetic powders (dispersing agents) such as fatty acids (JP-B-54-7074, the term "JP-B" as used herein means an "examined Japanese patent publication"), and the method of adding the above-described fatty acids to the ferromagnetic powders during production.
However, studies by the present inventors have revealed that the dispersed state of the ferromagnetic powders was not fully improved in all cases even if the above-described methods were employed.
For example, the ferromagnetic powders surface treated by using the above-described silane coupling agents are usually improved in their stability of the dispersed state in the magnetic layers, but with the drawback that compatibility with resin components sometimes decreases. Therefore, the dispersed state of the ferromagnetic powders in the magnetic layers is not fully improved in all cases.
The above-mentioned fatty acids usually contained in the magnetic layers of the magnetic recording media as lubricating agents also can have dispersing action on the ferromagnetic powders. However, when the fatty acids are specifically used as a dispersing agent, sufficient dispersing effect can not be obtained without using the fatty acids in relatively larger amounts than if the fatty acids were compounded in the magnetic layers as the lubricating agent instead. The use of such large amounts of fatty acids causes the problem of plasticizing the binders.
On the other hand, the surface smoothness of the magnetic layers is also improved with the improvement of the dispersibility of the ferromagnetic powders by the methods as described above. For this reason, the coefficient of friction of the magnetic layer surfaces increases, which results in a tendency of the magnetic layer to deteriorate in running properties and running durability.
JP-A-63-42025 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the magnetic recording media containing benzoic acid and salts thereof as dispersing agents which can prevent the plasticization of the binders induced when the above-described fatty acids are used. As to these magnetic recording media, the dispersibility and the plasticization of the binders are satisfactory, but the running properties and running durability are not completely satisfactory.
In order to address the above shortcomings of the conventional methods, the present inventors have already proposed to add organic phosphorus compounds such as phosphoric acid substituted by an alkyl or aryl group and salts thereof to the magnetic layers (JP-A-1-189025). The dispersibility of the ferromagnetic powders is significantly improved by this proposal, and the electromagnetic characteristics and the running durability are also improved thereby.
In some cases, however, even these improvements in properties are not necessarily sufficient under severe conditions such as the circumstances of high temperature and high humidity.
Further, JP-A-62-134819 discloses a magnetic recording media exhibiting excellent running durability and having excellent electromagnetic characteristics such as high S/N ratio. This media are obtained with binders containing vinyl chloride, a hydroxyl group-containing monomer and a polar group-containing monomer or copolymers containing vinyl chloride, vinyl acetate, a hydroxyl group-containing monomer and a polar group-containing monomer as constituting units. Further, the hydroxyl groups of this hydroxyl group-containing monomer are not directly combined with the vinyl groups and 50 to 95% by weight of the polar groups are epoxy groups. These binders improve the electromagnetic characteristics of the magnetic recording media.
However, the magnetic recording media obtained by this method are also insufficient in running durability and unsatisfactory in electromagnetic characteristics such as their maximum output level and sensitivity.
Also, the magnetic recording media are known in which vinyl chloride resins having epoxy groups are used as binders (JP-A-62-175928 and JP-A-63-166010), but these media also have problems in durability, for example.